This invention relates to semiconductor matrix types of sensors for detecting very small surface pressure distribution. More particularly, the invention relates to semiconductor elements for detecting minute distances between fine pressure distributions, such as might occur responsive to the pressure differentials resulting from the valleys and ridges of a finger print.
The following is a description of conventional finger print detectors or readers. In one apparatus, a finger tip is brought into contact with one surface of a prism which is irradiated by light, with the reflected light reverberating on a surface of a photo-detector element such as a CCD. As a result, a finger print pattern is detected in accordance with an output signal from the photo-detector element. However, this optical system is susceptible to adverse influences such as those caused by sweat and moisture.
One cause of a false reading, might be a residue from the sweat of a person who previously measured his finger print may still remain on the surface of the prism to cause measurement errors, resulting in either inconvenience or false readings. In greater detail, the residue may absorb light irradiated from outside for measurement of a finger print. Because of the residue, there is little reflection of the light from the surface of the prism, resulting in failure of measurement of the finger print.
There may also be many other causes of false readings. A failure of the measurement of a finger print may not only be due to the residue from the sweat of a person who previously measured his finger print, but also may be a result of moisture from anywhere, such as ambient humidity, rain, or the like. Dryness may also cause false readings due to a failure of the measurement to correctly detect a total reflection of the light from the surface of the prism. Therefore, a finger print pattern cannot always be accurately detected by a light reading method.
In addition, this light reflection method requires a high power consumption; therefore, it is not suitable for outdoor measurements by battery powered equipment.
The inventor has previously described finger print detecting apparatus (Japanese Patent Laid-Open Application No. 63-204374) for reading finger prints responsive to pressure or contact differentials of surface pressure applied by ridges on the finger tip. According to this pressure reading apparatus, a change in the conductivity on a surface is caused by a difference between the top ("ridges") and bottom ("valleys") of the skin surface on the tip of a finger. The conductivity is detected by ON/OFF states responsive to using a combination of a scanning electrode matrix and conductive rubber having a conductivity which changes in accordance with pressure. This pressure reader is free from for the above-described problems associated with light reflection.
The inventor has also described a surface pressure distribution detecting element in Japanese Patent Application No. 2-179735 (now U.S. Pat. No. 5,079,947). In this element, scanning electrodes extending in one direction are formed on a hard substrate. Resistance films are formed on the scanning electrodes which are spaced apart by a predetermined pitch (50 to 100 .mu.m). A flexible film has a lower surface extending over the scanning electrodes in a direction perpendicular to the direction in which the electrodes extend. A finger tip is pressed against the upper surface of the flexible film. As a result, the total resistance of the resistance films between the electrodes, which are perpendicular to each other, is changed in accordance with an areas in which the ridges of the skin surface are brought into contact with the resistance films.
The surface pressure distribution detecting element of U.S. Pat. No. 5,079,947 is free from the adverse influences caused by sweat and moisture. However, the described element has the following material, structural, and manufacturing problems. (1) as a practical matter, it is difficult to find a durable flexible film material which can accurately transmit the pressure distribution of the skin surface and which can assure a deposition of the scanning electrodes by an etching process or the like. (2) In the manufacturing process, it is very difficult to position the scanning electrodes to be exactly perpendicular to each other. (3) The pressure sensitivity of the element varies depending on the incremental surface positions that receive the pressure.